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1. Field of the Invention
The present invention relates to modems and more particularly to the electrical operating characteristics thereof.
2. Description of Related Art
Countries throughout the world impose various different standards for modem operating characteristics/ranges. The United States, for instance, expects a linear current-voltage curve, to provide a relatively constant impedance, regardless of the particular impedance. Japan, in contrast, requires an impedance of no greater than 300xcexa9. (If the device has an impedance of 100xcexa9, it might draw 30 mA, whereas, if the device has an impedance of 300xcexa9, it might draw 10 mA.) In still further contrast, Europe (the CTR21 spec) does not restrict impedance but specifies that (i) current cannot exceed 60 mA (upon rising to 60 mA, the voltage can skyrocket, but the current must not rise further) and (ii) if the device draws more than a certain current, it must not exceed a certain voltage. Still other countries might specify different types of operating characteristics.
Due to the variations in modem operating characteristics required throughout the world, manufacturers of modems have been unable to define a single universal device that will comply with all operating specifications. For instance, while the slope of the linear curve required by the United States can be varied so as to comply with Japan""s maximum 300xcexa9 specification, the linear curve cannot be modified so as to comply with Europe""s non-linear operating characteristic.
As a result, modem manufacturers have been forced to design and build several different flavors of a given version modem, where one flavor complies with the operating characteristics of one or more countries, another flavor complies with the operating characteristics of another country, and so forth.
A further difficulty arises from the possible variation in line current in a given country and a variation in line voltage depending on where the modem is positioned in relation to the telephone company central office. As an example, if a modem is located thousands of meters from the central office, the line impedance could result in a substantial voltage drop by the time the current reaches the modem. In contrast, if the modem is located only several meters from the telephone company central office, little voltage drop will occur on the line. As a result, a modem that might operate in the required range at one location in a given country may operate outside of the range at another location in the same country.
In view of the foregoing, the present inventors have discovered a need for a method and system to allow a modem to operate under a variety of conditions, so as to reduce or eliminate the need to produce multiple variations of a given modem.
The present invention provides a system for automatically adapting modem operating characteristics to meet a variety of specifications and line conditions. The invention is usefully employed when a modem is going off hook, before the modem enters a conventional handshaking state or other form of communication over the telephone line to which it is connected.
According to one aspect, the modem is provided with a line interface circuit and control logic. When the modem is first used, a user may input an indication of the country or region of operation of the modem. For instance, the user may indicate through a graphical users interface that the modem is being used in a particular country. Provided with this indication, the control logic may load into memory an indication of the location of operation, for use in adapting the operating characteristics of the modem to that location.
According to another aspect, the line interface circuit is electrically adaptable to meet desired and required operating characteristics. The circuit may, for instance, have selectively modifiable impedance and current segments. The control logic may cause these segments to change states so as to change the overall operating characteristics of the modem.
In this regard, if the modem had a way to know the voltage provided by the central office and the line impedance (along the telephone line between the central office and the modem), then the modem could adapt its operating characteristic as presently contemplated to interface with the telephone line in a way that would comply with applicable specifications. In particular, by knowing the supply voltage and the line impedance, control logic in the modem could readily compute the line voltage at the modem given various levels of modem impedance. Thus, the control logic could then adjust the modem impedance so as to comply with the applicable specification. In most cases, however, it is not possible to directly measure the actual line impedance, or at least to do so quickly enough to reach an acceptable operating state within an acceptable timeframe. (The European specification, for instance, requires a modem to reach the specified operating range within 20 milliseconds).
Thus, rather than actually measuring the line impedance, an exemplary embodiment of the invention starts by assuming what the line voltage will be. In particular, when the modem is about to go off hook (e.g., in response to an incoming or outgoing call), the control logic sets (or prepares to set) the off hook operating characteristics of the line interface circuit to a state that would be acceptable under the applicable specification, given some assumption of what the actual line voltage will be. The off hook operating characteristics are one or more electrical characteristics that will be exhibited by the modem (from the perspective of the telephone line, for instance) upon going off hook. These characteristics may include impedance, voltage, current or some combination of these or other measures. The line interface circuit may include a bridge circuit to extract a DC signal from the telephone line, so that the characteristics at issue may be substantially or completely DC characteristics.
Upon going off hook, the control logic may then detect the actual line conditions at the modem, such as the actual line voltage and current being drawn by the modem. With this information, the control logic may then determine whether the actual line conditions are (i) sufficient to allow operation of the modem and (ii) in compliance with the applicable specification. If the line conditions do not satisfy either or both of these criteria, then the control logic may adjust the state of the line interface circuit in an effort to change the operating characteristics of the modem to an acceptable state.
It is possible, for instance, that the system might assume that the line impedance is 1 kxcexa9, but the line impedance may actually be 3 kxcexa9. As a result of the assumption, the line voltage drop will be higher than assumed, so (assuming a constant modem impedance) the line voltage actually provided to the modem will be lower than assumed, resulting in the modem drawing less current than expected. If the voltage is too low for the modem to operate, or the current is too high for compliance with the applicable specification, or if otherwise desired, the system may rectify the situation by adapting the electrical operating characteristics of the modem. In particular, the system may adjust the modem impedance so as to draw less current, and to thereby decrease the line voltage drop and consequently increase the voltage across the modem.
According to another aspect, the line interface circuit may be adapted further to help ensure that the power supplied to components of the modem (such as components of the line interface circuit) does not exceed ratings. In doing so, again, the control logic may be cognizant of the operating range and applicable specification(s), so as to help ensure that the modem operates acceptably.
Thus, in one respect, the invention may take the form of a telephone device such as a modem, telephone, answering machine or fax machine. The telephone device includes a circuit that can be interconnected to a telephone line. The circuit may be selectively configured so as to present a desired off-hook impedance from the perspective of the telephone line. For this purpose, the circuit may include a gyrator circuit with selectively adjustable impedance (e.g., a transistor controlled impedance) and a loophold circuit with a selectively adjustable characteristic such as current or voltage (either correlating in turn with impedance).
The telephone device further includes a controller coupled with the circuit. The controller includes logic to determine an electrical characteristic of the telephone line (such as a line voltage received by the telephone device, and/or a line current drawn by the telephone device) and to responsively adjust the off hook impedance of the circuit so as to bring at least one electrical operating characteristic of the telephone device within an acceptable range. In particular, the controller may alter the voltage to the base of a transistor in the gyrator circuit so as to change the gyrator impedance, and the controller may change the current drawn by the loophold circuit and/or the voltage consumed by the loophold circuit (either correlating with a change in impedance).
The acceptable range within which the controller brings the circuit may be imposed by a government specification (or, equivalently, any type of regulatory or industry standard specification or the like). Alternatively or additionally, the acceptable range could be that range which would be required in order to allow at least some portion of the telephone device to function. For instance, the controller might require a certain minimum voltage level in order to function. So the acceptable range might be the range suitable to supply that minimum voltage level to the controller.
In another respect, the invention may take the form of a modem that can be adapted for use under a variety of operation specifications (e.g., government specifications as described above). The modem may include a circuit for interfacing with a telephone line, and the circuit may have at least one selectively adjustable electrical characteristic. In turn, the modem may include control logic coupled with the circuit and arranged to measure an electrical characteristic on the telephone line and responsively adjust an operating characteristic of the modem.
For instance, if the control logic determines that the current being drawn by the modem exceeds that allowed by the specification imposed by the country in which the modem is located, the control logic may raise the off hook impedance of the circuit so as to reduce the line current to the modem. In that way, the control logic can adaptively bring the modem into compliance with the specification.
The modem could also include a set of logic for receiving a user-provided indication of the location of operation of the modem. The user might provide this information during installation of the modem, for instance. The control logic may then determine one or more aspects of the applicable specification for that location based on the supplied indication.
In yet another respect, the invention may take the form of a method for adaptively configuring an operating state of a modem. The modem may have a circuit for electrically interfacing with a telephone line. The circuit may define an impedance with respect to the telephone line, and the circuit may be selectively switchable between an off hook state with respect to the telephone line and an on hook state with respect to the telephone line.
The method can include the steps of first setting an off hook impedance of the circuit and then, upon going off hook, measuring the line voltage at the modem and adaptively adjusting the off hook impedance of the modem if the line voltage is below some predefined threshold level.
In turn, the method can include measuring (computing) the line current being drawn by the modem and determining whether that line current exceeds a predefined threshold current level (such as a maximum current level set by a regulatory specification for instance). If so, the method might then include modifying the impedance of the circuit further so that the line current does not exceed the threshold.
Further, the method can include determining whether the off hook impedance exceeds (is greater than or less than) some predefined limit (such as a threshold set by a regulatory specification for instance). If so, then the method may include modifying the off hook impedance further so that it does not exceed the limit.
These as well as other features and advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.